1. Field of the Invention
The present invention relates to a racket for tennis and the like and more particularly to a racket having an improved grommet mounted in string holes formed through a string-stretching part to increase the sweet area of a ball-hitting face of a racket frame and improve the vibration-damping performance thereof.
2. Description of the Related Art
As shown in FIG. 8, in a conventional racket frame, string holes 2 are formed in penetration through a string-mounting part 1 constituted of a circular arc-shaped frame surrounding the ball-hitting face, with the string holes 2 disposed vertically to tangents to the string-stretching part 1. In this method, it is easy to use a drill in forming the string holes 2 in the direction from the inner peripheral side 1a of the racket frame 1 to the peripheral side 1b thereof. Further this method allows a required length of a string S to be short.
The strings S to be tensionally mounted on the racket frame are composed of longitudinal strings tensionally mounted through the string-stretching part 1 with the longitudinal strings disposed in parallel or approximately parallel with the longitudinal axis of the racket frame drawn from the top of the ball-hitting face to the center of the grip and transverse strings tensionally mounted through the string-stretching part 1 with the transverse strings disposed orthogonally to the longitudinal strings.
Except the longitudinal string passing through the top of the racket frame 1 and the transverse string passing through both sides of the head part having the longest width, the string-stretched direction is not coincident with the direction in which the string holes 2 are formed in penetration through the racket frame 1. Because the string S is bent in contact with the inner opening 2a of the string hole 2 disposed on the inner peripheral surface of the racket frame 1, the effective length of the string S is equal to the length between the opposed inner peripheral edges of the frame 1 surrounding the ball-hitting face.
The sweet area of the racket frame can be increased by increasing the effective length of the string. Therefore proposals for increasing the effective length of the string have been hitherto made.
For example, as shown in FIG. 9, in the construction disclosed in U.S. Pat. No. 2,534,963 (patent document 1), the inner diameter of the string insertion hole 4b inside the tubular portion 4a of the grommet member 4 to be mounted on the racket frame 3 is set larger at the inward open portion 4c than at the outward open portion 4d to allow the movability of the string S to be large inside the racket frame 3.
As shown in FIGS. 10A and 10B, in the construction disclosed in Japanese Patent Application Laid-Open No. 2004-105558 (patent document 2), the string protection member 5 mounted on the racket frame is constructed of the soft grommet members 6 inserted into the string holes of the racket frame and the belt-shaped member 7 layered on the periphery of the soft grommet member 6. The belt-shaped member 7 has the long and narrow slits 8 communicating with the insertion holes 6a of the soft grommet member 6 respectively and crossing the insertion holes 6a respectively in the ball-hitting direction. In this construction, owing to a stress applied to the string S when a ball is hit, the string S slides on the slit 8 in the ball-hitting direction.
In the grommet member 4 of the patent document 1 and the string protection member 5 of the patent document 2, because the deformation support of the string S moves from the inward side of the racket frame to the outward side thereof, it is conceivable that the effective length of the string increases and thus the sweet area increases.
But it is conceivable that in the grommet member 4 of the patent document 1, the range of contact between the string S and the grommet member 4 is short, and the vibration of the string S is insufficiently damped. Thus a player has an unpleasant feeling when the player hits a ball.
In the string protection member 5 of the patent document 2, because there is little contact between the string S and the grommet member 6, it is difficult to damp the vibration of the string S. Thus the player feels vibration unpleasantly when the player hits the ball. Because the string S slides on the slit 8 when the player hits the ball, the string S has a low degree of durability owing to the sliding contact between the string S and the slit 8. Further the position of the string S changes each time the player hits the ball. Thus the speed of the ball and the player's feeling vary each time the player hits the ball. Further because the grommet member 6 and the belt-shaped member 7 are made of different materials, the string protection member 5 is assembled at a low operability and liable to be defectively mounted on the racket frame.
Patent document 1: U.S. Pat. No. 2,534,963
Patent document 2: Japanese Patent Application Laid-Open No. 2004-105558